The use of composite materials is commonplace in today's manufacturing industry. Composite materials advantageously display certain desired physical and/or chemical properties different from the constituent materials. Recent advances in materials science have included development of polymer nanocomposites (PNCs). In the broadest sense, PNCs are comprised of a polymer matrix reinforced with nanoparticles having dimensions less than one hundred nanometers, but often in the range of one to fifty nanometers.
PNCs differ from conventional composite materials due to, among other features, a high surface area to volume ratio between the polymer and the nanoparticles. For example, the total surface area in a unit volume increases 1,000,000 times when the particle size is decreased from one millimeter to one nanometer. As a result, a relatively small amount of nanoscale reinforcement can have an observable effect on the macroscale properties of the composite. In other words, the nanocomposite (NC) properties are drastically increased at low concentrations of nanoparticles (NPs), generally 0.5-5.0 percentage by weight (wt %). For example, Young's modulus and yield strength are doubled at 1 wt % NPs in carbon nanotube/epoxy NCs compared to neat epoxy. One of the most important properties affecting NCs characteristics is maximal interfacial stress transfer between the polymer matrix and the NP surface. This characteristic is strongly dependent on the degree of dispersion and orientation of the NPs in the polymer matrix.
Incorporation of high aspect ratio nanoparticles (HARNPs), or nanoparticles with an aspect ratio greater than 100, into a polymer matrix can significantly increase mechanical properties such as elastic modulus and tensile strength. Additional enhanced properties may include gas permeability, fire retardancy, transparency, and electrical and thermal conductivity, magnetism, shape recovery, wear resistance, corrosion resistance, permeation resistance, self-healing, anti-lighting, conductance, photoluminescence and electroluminescence. For example, carbon nanotubes (CNTs) improve the electrical and thermal conductivity of the composite. Due to such extraordinary and desirable improvement in the properties of such composites, PNCs are used in demanding applications such as aerospace, automotive, electronics, computer technologies, and the like.
When properly dispersed, HARNPs (e.g., nanometer-thin platelets, such as clays to and graphene sheets, or nanometer-diameter cylinders, such as CNTs) interact with relatively more of a polymer chain than lower aspect ratio NPs in a unit volume of NCs. By contrast, low aspect ratio NPs (e.g., nanorods, polyhedral oligomeric silsesquioxanes (POSS), silica spheres) have fewer surface interactions to break, resulting in poorer performing systems. Therefore, higher energy is required to break HARNP-PNCs systems than low aspect ratio NP-PNCs systems. The nanosphere represents a low aspect ratio NP while the nanoplatelet is a high aspect ratio NP. Expanded polymer chains interacts with the HARNPs with much fewer larger polymer chains than the low aspect ratio NPs.
Agglomeration of HARNPs reduces the effective aspect ratio of the nanoparticles and available surface for interaction. For example, the aspect ratio of an agglomeration containing 100 nanoplatelets is 1 while for a single nanoplatelet is 100. Further, the total surface area of the individual platelet system may be increased by 34 times over that of the agglomeration. The increased surface area produces a significant increase in platelet-polymer interactions, resulting in improved performance with only a small percent of NP addition. Therefore, obtaining complete dispersion becomes important in maximizing PNC performance.
The polymer matrix and nanoparticles need to favorably interact with each other at their interface, which plays a crucial role for mechanical properties. A central issue is that most polymer matrices and NPs are not compatible with each other. In order to facilitate compatibility, NPs need to be functionalized with surfactants that are compatible with both the NP and polymer matrix. The functionalization with surfactants, however, can have disadvantageous environmental impacts.
In general, there are four critical requirements for effective nanoparticle reinforcement of NC materials: 1) high aspect ratio of NPs, 2) interfacial compatibility between NPs and polymer matrix, 3) complete uniform dispersion, and 4) controlled orientation. As previously discussed, higher aspect ratio NPs exhibit the best reinforcement effect. Interfacial compatibility is vital to achieve effective load transfer between the NPs and the polymer matrix. For example, HARNP and the polymer matrix need to be compatible with each other in terms of surface wettability. Complete uniform dispersion of NPs results in higher surface area and a greater aspect ratio. Orientation of the HARNPs is critically important to enhance mechanical properties such as tensile modulus and strength compared to the mechanical properties obtained from NCs with only dispersion. Additionally, orientation can result in new and controllable anisotropic mechanical and functional properties in PNCs.
Properties of NCs are significantly affected by the fabrication method. For example, PNCs are commonly fabricated using melt mixing and solution mixing methods. The melt mixing methods are attractive due to being environmentally friendly, inexpensive, and continuous, but these methods lack the ability to disperse or orientate NPs, thus requiring the need for additional processing. The solution mixing methods are discontinuous and environmentally unfriendly. Examples of the melt and solution mixing methods include single and twin-screw extruders, two and three roll milling, ultrasonication mixing, solution mixing, water injected melt mixing, high shear mixing, in-situ polymerization, melt dispersion, batch mixing, and mechanical stirring. However, these fabrication methods have not been able to achieve NCs with the extent of theoretically predicted superior properties due to inadequate dispersion of agglomerated NPs as well as inadequate orientation of the NPs in the NC.
The two-roll mill is not suitable for production at industrial scale due to its difficulty to scale up and continuously processing with thermoplastic NCs. Single-screw extruders cannot provide sufficient dispersion in nanoscale even at low concentrations of weight percent of NPs, because of its low shear rate. Despite literature indicating that twin-screw extruders are the best dispersing machines among melt mixing techniques, the twin-screw extruders can only partially exfoliate or only disperse nanoparticles within polymer matrix. The twin-screw extruder is extensively used for mixing. Thus, the single screw extruder is not an efficient dispersive mixer because of insufficient high shear regions. In a single screw extruder the high shear region is created only within screw flight clearance. For instance, in a single flighted single screw extruder rotating at 100 revolutions per minute, 65.4% of molten PNC does not pass over the flight, 27.7% passes once, 5.9% twice, and 0.8% three times. However, to achieve properly dispersed NCs, the molten PNCs should encounter at least twenty passes through high shear zones.
PNCs have been fabricated mainly by melt mixing, in-situ polymerization, solution mixing, and ultrasonication, depending on polymer and NPs properties. The critical polymer properties included polymer solubility, viscosity of molten polymer, and polymer type such as thermoplastic or thermoset. In-situ and ultrasonication dispersion methods are not desirable from an environmental point of view. Melt mixing does not require an additional processing step, its simplicity to facilitate large scale production for commercial applications, and it is environmentally friendly by not requiring a solvent. Melt mixing dispersion levels are lower than those obtained through ultrasonication and in-situ polymerization because of the insufficient shear rate. Nevertheless, as mentioned earlier, these methods cannot work with high viscosity molten polymers. The NCs that are fabricated by current methods have not exhibited the extraordinary mechanical and conductive properties due to poor dispersion of the NPs.
During a typical melt mixing operation, dispersion occurs when hydrodynamic shear forces overcome the cohesion forces between the NPs. The cohesive force could be comprised of Van der Waals forces, electrostatic forces, and/or magnetic forces. However, the hydrodynamic shear force is only suitable for dispersion of NPs agglomerations when viscosity is high.
There are two types of agglomeration breakup mechanisms: rupture and erosion. The rupture mechanism occurs by splitting of the agglomeration into fewer numbers of aggregates. The rupture process requires relatively high shear forces, the hydrodynamic force needs to be at least five times higher than the cohesive force of the NPs. The particle erosion process is characterized by a continuous peeling of primary particles from the outer agglomerate surface. The particle erosion process occurs at lower hydrodynamic shear forces of two times the cohesive force depending on agglomerate behavior.
In order to achieve the predicted extraordinary mechanical and functional properties of PNC materials, the nanomaterials should be exfoliated, dispersed, and oriented within the polymer matrix during processing. Dispersion and orientation of nanomaterials within the polymer matrix can generally only occur after exfoliation of the HARNPs. Effective dispersion of the nanoparticles within the matrix is essential to ensure consistent and predictable properties throughout the composite. Therefore, a need exists in the art for a system that is capable of simultaneous dispersion and orientation of nanoparticles within a polymer matrix at high temperatures without solvents.
Dispersion of NPs within the polymer matrix is a complicated process because of the high viscosity of polymers, interfacial surface incompatibility between polymer matrix and NPs, and NP agglomeration. The high viscosities of molten polymers result in laminar creeping flows in PNCs processing. And if turbulent flow is created polymer degradation will occur due to viscous dissipative heating and the difficulty in removing the heat from the system. On the other hand, the high viscosity of molten polymer enables greater transfer of the shearing forces to the agglomeration.
Furthermore, controlling placement (i.e., orientation) of the dispersed NPs can be obtained only after proper dispersion. Such orientation methods commonly known in the art include shearing drawing, melt and electrospinning, equal channel angular extrusion, drawing, filtrating, applying magnetic and electric field, shear flow, spin coating, gas-liquid interfacial flow. However, none of these methods have produced a NC with properties close to the theoretical mechanical and functional property limits. For example, the drawing and the various spinning methods produce only one-dimensional materials such as fibers. Other methods such as electric and magnetic fields and shear flow orientation methods can produce two dimensional films or three dimensional bulk NCs; however, these fabrication methods have not yet reached practical usage. The shearing orientation method is the most promising method when compared with aforementioned orientation methods because it does not require special functional properties of the NPs. For example, a key factor for shear orientation is the high aspect ratio of the NPs while electrical and magnetic field orientations require anisotropic electrical and magnetic properties of the NPs, respectively.
Orientation does not require high energy as is necessary for dispersion. However, orienting HARNPs within a polymer matrix requires fully-developed, steady, laminar, shear flow (FDSLSF). Achieving FDSLSF is complicated due to surface roughness at the nanoscale. Therefore, a further need exists in the art for a system that is capable of generating FDSLSF to orient the PNCs.
A high degree of orientation and distribution of dispersed HARNPs throughout the matrix can give the greatest strength and stiffness along long axial direction, but the material is much weaker in the other directions. If the HARNPs are randomly oriented (i.e., isotropic) the mechanical and physical properties will be intermediate. Easier transfer of electrical and thermal energy will occurred along oriented direction of HARNPs when all HARNPs are oriented in same direction within polymer matrix. Achieving consistent uniform dispersion, alignment, and orientation of the HARNPs will allow optimal property improvement. Controlling the alignment and orientation of HARNPs in the polymer matrix can be tailored to best fit the NCs desired application.
It has been established that HARNPs are orientated along shearing direction in shear-induced flow because of the HARNPs anisotropic physical structure. Anisotropic properties of HARNP PNCs displayed very substantial physical effects in barrier, mechanical, and electrical properties. Generally, polymer nanoclay NCs show dramatic improvements of their barrier properties due to their tortuous gas diffusivity paths, known as Nielsen's theory. The barrier properties are enhanced when the exfoliated clay platelets are oriented. The oriented NCs have longer tortuous paths than randomly dispersed NCs.
The NC with oriented CNT always shows higher reinforcement along the oriented direction of the CNTs than randomly dispersed. Orientation of the HARNPs within polymer matrix exhibits enhanced tensile modulus and strength properties than the only dispersed HARNPs within polymer matrix. Similarly, it has been demonstrated that CNTs can dramatically enhance the electrical and thermal conductivities of polymers. The electrical and thermal conductivities along oriented direction are significantly higher than other directions.
Additionally, polymer chains are extended during the shear-induced orientation. Polymers with extended chains have denser packing than folded chains. Furthermore, the polymer crystallinity is increased due to polymer chain extension. Therefore, revolutionary progress in CNT application can only be realized when a technique is developed for the dispersion of the entangled CNTs and then controlling the dispersed CNTs orientation within the PNCs. The improved crystallinity leads to high strength, good toughness, high stiffness, low gas permeability, a higher melting point, good fatigue life, good abrasion resistance, and enhanced chemical resistance.